For many years, various forms of supports have been used for maintaining production decks above the water in the development of offshore oil fields. The most common support structure in service today is the steel jacket made of tubulars and piled to the sea floor. Recently, gravity structures have been built from steel-reinforced concrete and put in service at offshore oil field locations. These concrete gravity structures usually have a caisson at their lower extremity that serves as a large base and also as a storage capacity for oil production. Both the piled-in-place steel structure and the floated-to-location gravity structure require offshore construction associated with the processing equipment and its allied services. This offshore construction is time-consuming and costly. If this construction can be done onshore, the oil production can be brought onstream with increased economy.
Jackup structures, which find extensive application in drilling operations offshore, have been used for short periods of time as production platforms. These jackup structures for drilling application have some shortcomings when used in production platform services. The fatigue life of the drilling jackup structures is not suitable for the lengthy service requirements for production operations. Likewise, most drilling-type jackup structures have very limited water depth capabilities, especially in areas of harsh environments such as the North Sea. The lack of oil storage in such structures may also be a limiting factor.
The offshore oil industry has identified the need for a production platform-type structure that meets the following criteria:
A structure that could be floated to the offshore location completely outfitted so that predrilled wells can be brought onstream with minimum delay. PA1 A structure that can provide oil storage for intermittent tanker loading when needed. PA1 A structure that has an in-service life of at least 20 years. PA1 A structure that will accommodate water depths of at least 400 feet in areas such as the North Sea and tolerate the deck loads typical of production operations. PA1 A structure that has much less catastrophic impact damage susceptibility in contrast to the susceptibility of the drilling-type jackup. PA1 A structure which includes a tower with which to provide lateral support for the well conductors. PA1 A structure that can be fabricated with support legs in a substantially horizontal position, the legs subsequently being elevated to their permanent vertical position by the use of the jacking mechanism to the subsequently utilized between the production deck and the support legs.